A conventional fuel injection nozzle of a diesel engine using light oil as fuel includes, for example, a needle valve 2 as shown in FIG. 9. As shown in FIG. 10, in a state where an outer peripheral surface of a tip part 21 of the needle valve 2 is seated on a valve seat part 33, communication of light oil fuel to a fuel injection port 31 of a nozzle body 3 is blocked and a fuel injection nozzle 1 is put in a valve closed state (for example, see JP-A-11-13576). The tip part 21 of the needle valve 2 has such a shape that the ratio of a shaft diameter L1 and a seat diameter L2 of a seat part 211 coming in contact with the valve seat part 33 becomes L2/L1=0.60 to 0.75. Besides, it has such a shape that the ratio of the seat diameter L2 and a center diameter L3 for regulating a minimum flow path area at full lift of the needle valve 2 becomes L3/L2=0.35 to 0.60. As shown in FIG. 11, the needle valve 2 is lifted in a direction denoted by reference character B, the flow path is constructed of a space produced by separation of the tip part 21 of the needle valve 2 from the valve seat part 33, and the fuel injection nozzle 1 is put in a valve open state. The needle valve 2 is regulated so that a minimum flow path area d at a time when it is lifted by a lift amount denoted by reference character D and is put in a full lift state becomes an optimum flow path area by the center diameter L3.
High pressure light oil fuel supplied from a not-shown injection pump, a common rail and the like flows into the inside of the valve seat 33, and is sprayed into a combustion chamber of the diesel engine from the fuel injection hole 31 as indicated by reference character C. The high pressure light oil fuel sprayed into the combustion chamber is burned by spontaneous ignition due to mixture with high temperature and high pressure air.
As stated above, the combustion of the diesel engine is performed in such a way that the fuel is sprayed into the high temperature and high pressure air, and the spontaneous ignition is caused by the mixture of the air and the fuel. However, atmospheric pollution by the diesel engine becomes a problem in recent years, and as measures against the atmospheric pollution, attention is paid to using DME, whose exhaust is clean, as fuel instead of the light oil. Since the DME fuel has a high cetane number and compression ignition is possible, it can be used as fuel of the diesel engine. Besides, since smoke and SOx are not contained at all in the exhaust gas, it is harmless to human beings, and there is no influence on the destruction of the ozone layer and the greenhouse effect. Further, since it is relatively easily synthesized from various raw materials other than oil, it can be mass-produced at a low cost comparable to light oil.
Accordingly, from social demands such as the environmental problem due to the exhaust gas, the exhaustion of oil resources, and the necessity of fuel diversification against an unexpected oil crisis, there is a high possibility that it is realized in near future that traveling and possession of an existing diesel engine vehicle using light oil as fuel is prohibited by regulations. Besides, at the present moment, except for DME, there is nothing more valuable than DME as an alternate fuel of a diesel engine substituting for the light oil, and a diesel engine system of the DME fuel is currently at an advanced stage of development, and accordingly, it can be said that the light oil is substituted by DME as the fuel of the diesel engine before too long.
However, as described above, although DME has many merits as the fuel of the diesel engine, as compared with the light oil, energy obtained from a same amount of fuel is low, and accordingly, when the amount of fuel injection is the same as that in the case of the light oil, an engine output becomes lower than that in the case of the light oil. Thus, a conventional diesel engine using light oil as fuel can not be used directly as a diesel engine using DME as fuel. In the case where a diesel engine using DME as fuel is designed and manufactured from the first, it is sufficient if the amount of fuel injection is set so that a specified engine output can be obtained by the DME fuel. However, in an existing diesel engine vehicle using light oil as fuel, in case the mounted whole diesel engine must be exchanged with a diesel engine of DME fuel, it takes very high cost, labor, and time, and this can not be said to be realistic. Accordingly, even if a diesel engine vehicle of DME fuel is put to practical use, the replacing of existing light oil fuel diesel engine vehicles is not smoothly carried out, and by that, the spread of the diesel engine of DME fuel is much delayed, and there is a fear that measures against the worrying destruction of the environment and global warming are delayed, and the exhaustion of oil resources is hastened.